The present invention generally pertains to injection molding of hollow plastic products and is particularly directed to systems and methods for reducing the required mold-cavity clamping force and for controlling the wall thickness of a molded product having an end wall and a cylindrical or approximately cylindrical side wall, wherein the end wall includes a fringe region bordering the sidewall.
One prior art, method for injection molding such a product includes the steps of
(a) forming a mold cavity by combining a core section for forming the inside surface of the product with a cavity section for forming the outside surface of the product, wherein the core section or cavity section includes a protractable and retractable ram-shaped contact element, with the end of the contact element that faces the mold cavity including a contact surface;
(b) protracting the contact element to a forward position so that the contact surface contacts the cavity section or core section of the mold cavity respectively;
(c) injecting a first quantity of plastic material into the mold cavity while the contact element is in the forward position;
(d) retracting the contact element to a rear position where the contact surface ceases to contact the cavity section or core section of the mold cavity respectively;
(e) injecting a second quantity of plastic material into the mold cavity while the contact element is in the rear position.
Also in the prior art, the wall thickness of an injection molded plastic product is controlled by a method including the steps of
(a) forming a mold cavity by combining a core section for forming the inside surface of the product with a cavity section for forming the outside surface of the product, wherein the core section or cavity section includes a protractable and retractable ram-shaped contact element, with the end of the contact element that faces the mold cavity including a contact surface;
(b) protracting the contact element to a forward position so that the contact surface contacts the cavity section or core section of the mold cavity respectively, to secure the core section in position in relation to the cavity section in order to impede movement of the core section in relation to the cavity section;
(c) injecting a first quantity of plastic material into the mold cavity while the contact element is in the forward position, whereby the wall thickness of the cavity is controlled because relative movement between core section and the cavity section is impeded;
(d) retracting the contact element to a rear position where the contact surface ceases to contact the cavity section or core section of the mold cavity respectively, while the injected first quantity of plastic material at least partly solidifies to impede movement of the core section in relation to the cavity section and thus control the wall thickness of the cavity during further injection of plastic material into the mold cavity; and
(e) injecting a second quantity of plastic material into the mold cavity while the contact element is in the rear position.